Acute respiratory distress syndrome and extracorporeal membrane oxygenation
Several articles have addressed important aspects of the acute respiratory distress syndrome (ARDS) and its treatment.
The incidence and outcome of ARDS in the era of lung protective ventilation have been evaluated by Villar and colleagues [
1] in a 1-year prospective, multicenter, observational study, under the acronym ALIEN (Acute Lung Injury: Epidemiology and Natural History). The study was conducted in 13 geographical Spanish areas, which serve a population of 3.55 million of adults. A total of 255 mechanically ventilated patients fulfilled the ARDS definition, representing an incidence of 7.2/100,000 population/year. Pneumonia and sepsis were the most common causes of ARDS. At the time of meeting ARDS criteria, mean PaO
2/FiO
2 was 114 ± 40 mmHg, mean tidal volume was 7.2 ± 1.1 ml/kg predicted body weight, mean plateau pressure was 26 ± 5 cmH
2O, and mean positive end-expiratory pressure (PEEP) was 9.3 ± 2.4 cmH
2O. Overall ARDS intensive care unit (ICU) and hospital mortality was 42.7% (95% CI 37.7–47.8) and 47.8% (95% CI 42.8–53.0), respectively, suggesting that despite the use of lung protective ventilation, overall ICU and hospital mortality of ARDS patients is still higher than 40%.
A couple of articles dealt with lung recruitment in patients with ARDS. Dellamonica et al. [
2] compared two methods to evaluate PEEP-induced lung recruitment, one based on the pressure–volume curve (considered as gold standard) and the other based on the bedside end-expiratory lung volume (EELV) measurement using the nitrogen washout/washin technique. PEEP-related strain was also assessed at two levels of PEEP, low and high. Recruitment assessed with the two techniques was well correlated. In addition, the ratio between PEEP-induced lung volume changes (ΔEELV) and functional residual capacity differentiated high from low recruiters (110 vs. 55%). Strain increased with PEEP and was larger in high recruiters.
Arnal et al. [
3] assessed the dynamics of lung recruitment and the hemodynamic changes during a recruitment maneuver (sustained inflation at an airway pressure of 40 cmH2O maintained for 30 s) in 50 patients with ARDS. Most of the recruitment occurred during the first 10 s of the recruitment maneuver (mean time constant for lung volume increase was 2.3 ± 1.3 s), while arterial pressure decreased significantly after 10 s. The authors concluded that a duration of 10 s may be recommended for a sustained inflation to achieve sufficient lung recruitment while preventing hemodynamic compromise. This article was discussed in an editorial by Marini [
4]. The protective ventilatory strategy may fail in the treatment of ARDS. Under this condition other therapeutic strategies can be attempted.
Charron and colleagues [
5] reported the characteristics and prognosis of patients with severe lung injury (PaO
2/FiO
2 <100 mmHg after 24–48 h of mechanical ventilation) who are routinely ventilated in the prone position using a low stretch ventilation strategy. In this monocenter study, the authors provide data of 218 patients with ARDS admitted between 1997 and 2009. Of these patients, 57 (26%) were positioned prone because of a PaO
2/FiO
2 <100 mmHg after 24–48 h of mechanical ventilation. Prone sessions lasted 18 h/day, and 3.4 ± 1.1 sessions were required to obtain an FiO
2 <60%. The 60-day mortality was 19%, and death occurred after 12 ± 5 days. Logistic regression analysis showed that among the 218 patients, PP appeared to be protective with an odds ratio of 0.35 [0.16–0.79]. It was concluded that routine prone positioning is feasible in patients with a PaO
2/FiO
2 <100 mmHg after 24–48 h, and suggest that this strategy is protective and has a high survival rate.
In patients with ARDS, the combined use of high frequency oscillation (HFO) and tracheal gas insufflation (TGI) improves oxygenation versus standard HFO. Mentzelopoulos and colleagues [
6] hypothesized that HFO + TGI would reduce the amount of nonaerated lung tissue in the dependent lung. Authors studied 15 patients who had ARDS, and they were submitted to a whole-lung CT scan after lung-protective conventional mechanical ventilation (CMV) and after 45 min of HFO and 45 min of HFO + TGI. They found that HFO + TGI versus HFO and CMV resulted in a lower percentage of nonaerated lung tissue (mean ± SD, 51.4 ± 5.1% vs. 60.0 ± 2.5%, and 62.1 ± 9.0%, respectively;
p ≤ 0.04); this was due to HFO–TGI-induced recruitment of nonaerated tissue in the dependent and nondependent lower lung. In addition, HFO + TGI significantly improved oxygenation versus HFO + TGI. It was concluded that HFO + TGI improvement in oxygenation occurs through a mechanism of recruitment of previously nonaerated lower lung units.
The most severe cases of ARDS not responding to conventional and advanced treatments need to be approached through extracorporeal membrane oxygenation (ECMO).
In a interesting study Nair and colleagues [
7] describe the technical challenges, efficacy, complications, and maternal and infant outcomes associated with ECMO for severe ARDS in pregnant or postpartum patients during the 2009 H1N1 pandemic. This retrospective observational study, conducted on seven tertiary hospitals in Australia and New Zealand, studied 12 patients (7 pregnant and 5 postpartum). Their median [interquartile range (IQR)] age was 29 (26–33) years; 6 (50%) were obese. Two patients were initially treated with veno-arterial (VA) ECMO. All others received veno-venous (VV) ECMO with one or two drainage cannulae. ECMO circuit-related complications were rare, circuit change was needed in only two cases, and there was no sudden circuit failure. Bleeding however was common, leading to large volumes of packed red blood cell transfusions [median (IQR) volume transfused was 3,499 (1,451–4,874) ml] and was the main cause of death (three cases). Eight (66%) patients survived to discharge. The survival rate of infants whose mothers received ECMO was 71%, and surviving infants were discharged home with no sequelae. It was concluded that ECMO is effective, with outcomes comparable to those of non-pregnant patients and acceptable infant outcomes.
Another interesting study [
8] described the development and results of a retrieval program developed in New South Wales (NSW), Australia, to provide ECMO for the safe transport of adults with severe, acute respiratory or cardiac failure, from 1 March 2007 to 1 June 2010. Forty adult patients were retrieved on ECMO support (median age 34 years). The indications for retrieval were respiratory in 38 patients (of whom 16 were confirmed or suspected H1N1 cases) and cardiac in two patients. Two other patients died after referral but before ECMO support could be established.
Patients were transported by road (65%), medical retrieval jet (25%) and helicopter (10%). Thirty-four patients (85%) survived to hospital discharge. There were no deaths or major morbidity associated with these retrievals. It was concluded that severe respiratory failure, which was considered to preclude conventional ventilation for safe transfer to tertiary centers, was managed by an ECMO referral and retrieval program in NSW and had a high rate of survival. This program also enhanced the capacity of the state to respond to a surge in demand for ECMO support due to the H1N1 epidemic.
A multicenter Italian study [
9] reported the organization and results of a national ECMO network (ECMOnet) that was set up in view of the expected 2009 influenza A (H1N1) pandemic. The network consisted of 14 ICUs with ECMO capability and was set up to centralize all severe ARDS patients to the ECMOnet centers assuring safe transfer. Between August 2009 and March 2010, 153 critically ill ARDS patients (53% referred from other hospitals) were admitted to the ECMOnet ICU with suspected H1N1. Sixty patients received ECMO according to ECMOnet criteria (and 28 were transferred while on ECMO). Survival to hospital discharge in patients receiving ECMO was 68%. The length of mechanical ventilation prior to ECMO was an independent predictor of mortality. It was concluded that a network organization based on preemptive patient centralization allowed a high survival rate and provided effective and safe referral of patients.
Weaning
Patient-ventilator interaction during pressure support ventilation (PSV) and proportional assist ventilation plus [PAV+] were compared in a physiological study involving 11 patients difficult to wean [
10].
During three consecutive trials [a first trial of PSV (PSV1), followed by PAV+, followed by a second PSV trial (PSV2), with the same settings as PSV1], patients' mechanical and respiratory patterns were evaluated. Compared to PAV+, during PSV trials, the mechanical inspiratory time [Ti(flow)] was significantly longer than patient inspiratory time [Ti(pat)] (p < 0.05); Ti(pat) showed a prolongation between PSV1 and PAV+, and PAV+ significantly reduced expiratory trigger delays [delay(trexp)] (p < 0.001).
The portion of tidal volume (VT) delivered in phase with Ti(pat) [VT(pat)/VT(mecc)] was significantly higher during PAV+ (p < 0.01). The time of synchrony was significantly longer during PAV+ than during PSV (p < 0.001). During PSV, 5 patients out of 11 showed an asynchrony index (AI) greater than 10%, whereas the AI was nil during PAV+. The authors concluded that PAV+ improves patient–ventilator interaction, significantly reducing the incidence of end-expiratory asynchrony and increasing the time of synchrony.
Rozé et al. [
11] evaluated a method for setting neurally adjusted ventilatory assist (NAVA) during weaning, with the NAVA level adjusted to obtain 60% of the maximal diaphragmatic electrical activity (EAdi) measured during a spontaneous breathing trial (SBT). This method proved to be feasible and well tolerated, allowing for a progressive reduction of NAVA level during weaning.
A second study [
12] compared the effect of NAVA and PSV on patient–ventilator interaction in 22 intubated patients (COPD 36%). Patient–machine synchrony was better with NAVA, as demonstrated by the lower number of asynchronies per minute and the reduction in the number of patients with asynchrony index >10%. Improvement concerned particularly the inspiratory trigger delay, ineffective efforts, and premature and late cycling off. Finally, Tuchscherer et al. [
13] assessed the feasibility of a prolonged application of NAVA in patients with critical illness-associated polyneuromyopathy. As compared with healthy subjects, these patients showed a prolonged phrenic nerve latency and a decreased diaphragm compound muscle action potential. Nevertheless, NAVA could be applied in 13/15 patients for up to 3 days, with stable breathing pattern, hemodynamics and gas exchange while preserving respiratory drive.
Two articles dealt with the prediction of weaning outcome in mechanically ventilated patients. Zapata et al. [
14] evaluated the ability of B-type natriuretic peptides (BNP) to predict and diagnose weaning failure from cardiac origin. Thirty-two out of 100 patients failed a SBT, 12 because of heart failure and 20 because of respiratory failure. BNP and NT-proBNP were higher before SBT and increased significantly during SBT in patients failing because of heart failure. BNP performed better than NT-proBNP for prediction and diagnosis of weaning failure from cardiac origin.
Sellares et al. [
15] compared clinical characteristics and outcomes of patients with simple, difficult and prolonged weaning, and assessed predictors for prolonged weaning and survival in 181 mechanically ventilated patients. Patients with simple (43%) and difficult (39%) weaning had similar characteristics and outcomes, while those with prolonged weaning (18%) had a higher incidence of COPD, more complications and worst outcomes. Heart rate ≥105/min and PaCO
2 ≥54 mmHg during SBT predicted a prolonged weaning. In addition, reintubation and hypercapnia during SBT independently predicted a decreased 90-day survival. These findings were commented on in the editorial by Laghi [
16].
When the weaning fails patients may need to be tracheotomized. Fikkers and colleagues [
17] in a prospective randomized clinical trial involving 120 patients, compared two techniques of percutaneous tracheostomy: the guide wire dilating forceps (GWDF) and the single step dilatational tracheostomy (SSDT) technique. The early and late outcomes were analyzed. Sixty patients in each group were followed for up to 3 months after decannulation. Most of the reported complications in both groups were minor (58.3% in the GWDF group and 61.7% in the SSDT group). A trend was found towards an higher percentage of major perioperative complications in the GWDF group in comparison to the SSDT group [10.0 vs. 1.7% (
p = 0.06)], with a comparable long-term outcome.
Miscellanea
Carlesso et al. [
84] analyzed, with the help of a mathematical model, the effect of diluting human plasma with crystalloid solutions that have strong ion difference (SID). Their findings were that the baseline bicarbonate concentration will determine the pH response to crystalloid infusion. Thus, if the crystalloid equals baseline bicarbonate concentration, pH will not be altered at constant PCO
2, whereas it decreases if the SID is grater and decreases if SID is lower than baseline bicarbonate.
In an observational study, Bonizzoli et al. [
85] evaluated the rate of thrombosis associated with peripherally inserted central venous catheters (PICC) and central venous catheters in patients discharged from the ICU. Patients with PICC (
n = 114) had a significantly higher incidence of deep venous thrombosis than patients with central venous catheters (
n = 125) (27 vs. 10%), the majority of thrombosis occurring within 2 weeks after PICC insertion. Female gender and access through the left basilic vein were associated with a higher risk of thrombosis in the PICC group. The authors concluded that, in patients discharged from the ICU, routine ultrasound surveillance for the first 2 weeks after PICC insertion and preferential use of central venous catheters may be warranted.
De Keulenaer and coworkers [
86] investigated whether intra-abdominal pressure (IAP) assessed by the urinary bladder pressure correlated well with femoral venous pressure in 149 critically ill, mechanically ventilated patients. The bias between IAP and femoral venous pressure was −1.5 and precision was 3.6 mmHg, with large limits of agreement (−8.6 and 5.7). When IAP was above 20 mmHg, bias was 0.7 with a precision of 2.0 mmHg (lower and upper limits of agreement of −3 and 4.6, respectively). Excluding patients with abdominal compartment syndrome, a femoral venous pressure of 11.5 mmHg predicted intra-abdominal hypertension with a sensitivity of 85% and a specificity of 67%. The authors’ conclusions were that femoral venous pressure cannot be recommended as a surrogate measure for IAP measurement via the bladder, unless IAP is above 20 mmHg.
In a multicenter cohort study including data from 8,962 critically ill patients, Ho and coworkers [
87] assessed whether the intensity of smoking history has a dose-related effect on hospital mortality. Smokers were more frequently male and had a higher incidence of severe chronic cardiovascular, respiratory and liver diseases. The risk of requiring mechanical ventilation and dying in hospital was higher and ICU stay was longer in smokers than in non-smokers. After adjusting for other confounders, intensity of smoking history (measured in pack-years) remained significantly associated with hospital mortality in a relatively linear fashion.
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